Magnetic resonance (MR) imaging is an imaging method, which is used in many fields of medicine for examination and diagnosis. It is based on the physical effect of nuclear magnetic resonance. To this end, to acquire MR signals, a basic-field magnet applies a basic magnetic field within a region to be examined which aligns magnetic moments of nuclei, for example hydrogen nuclei H-1 or nitrogen nuclei N-14.
The irradiation of radio-frequency (RF) pulses enables the nuclear spins to be deflected or excited out of the aligned position parallel to the basic magnetic field, i.e. the rest position or another state. During relaxation in the rest position, a decay signal is generated, which can be detected as an MR signal by means of one or more RF receive coils. For example, selective dephasing and rephasing of the nuclear spins by suitably switched gradient fields can generate an MR signal. An effect of this kind is used in so-called gradient echo MR imaging sequences.
The application of a slice selection gradient during the irradiation of the radio-frequency pulses only excites nuclear spins in one slice of the object to be examined, in which the resonance condition is fulfilled due to the local magnetic field strength. Further spatial encoding can be performed by applying at least one phase-encoding gradient and one frequency-encoding gradient during the readout. This enables spatially resolved MR signals to be obtained from a number of slices of an examination subject. In this way, suitable imaging methods enable the provision of a 3-dimensional (3D) image of a specific area of the examination subject for purposes of diagnosis. Here, a typical spatial resolution of MR imaging can be, for example, 1 mm in all three spatial directions. A spatially extended imaging point of this kind is called a voxel.
For MR imaging, a patient is generally placed on a couch or a table in the interior of the main-field magnet. In addition, local RF coils are used to improve MR imaging, said coils being placed in the immediate vicinity of the patient. As a result, the imaging area contains not only the patient, but also other parts, such as, for example, the couch and the coils, which are made of a wide variety of materials. However, these materials can also be imaged because they comprise nuclei which are also used for MR imaging.
Imaging properties of materials located within the examination area used for MR imaging can give rise to artifacts in the MR images. Artifacts of this kind can result in an incorrect diagnosis or render the image unusable for purposes of diagnosis. Only relatively few materials are known which in empirical tests have reduced visibility in MR imaging. Since, in addition to reduced visibility in MR imaging, there are also further criteria which determine suitability for use in an MR system, for example no or low electrical conductivity and no or low magnetic susceptibility, the number of usable materials is limited.
Since significantly less-expensive plastic materials, for example, cannot be used, this may result in increased costs for the production of components to use in the MR system. In addition, it not possible to use, for example, soft and flexible plastic materials, such as those known from various aspects of daily life, since, as solid materials, these do not have reduced visibility in MR imaging. This can result in reduced comfort and restricted design freedom when using or producing components for use in an MR system. In addition, it is not possible to use materials that are particularly easy to produce or particularly robust or stable materials. This can result in reduced reliability or a reduced lifetime of the components to be used in the MR system.
For example, techniques are known from U.S. Pat. No. 7,604,875 B2 which enable the magnetic susceptibility of carrier materials to be matched to fixed predetermined values by the addition of paramagnetic and/or diamagnetic materials. However, the techniques disclosed therein relate to the reduction of a susceptibility mismatch, as a result of which the static magnetic field varies on a length scale of several centimeters and deviates from the desired value of the basic magnetic field. This can result, for example, in shifts or spatial domain distortions in MR images or have a negative influence on the quality of spectral fat saturation techniques. However, the visibility of the materials in MR imaging is not affected.